1. Field of the Invention
The present invention relates to methods for fabricating insulated, concentric tubular bodies and the bodies fabricated by such methods. Insulated tubulars or pipe of this type are sometimes referred to as "pipe-in-pipe" or "tube-in-tube" fluid conduits. Insulated, concentric tubular fluid conduits are employed in well completions, pipelines, refineries, and other processing installations.
2. Prior Art Setting of the Invention
The tube-in-tube insulated pipe employed in oil and gas wells is typically joined together end-to-end by threaded connections. In the case of some of the designs employed in oil and gas wells, the threads must be formed on the tubing after the concentric pipe bodies have been assembled. The typical insulated section employs a threaded external tube surrounding a shorter, smaller diameter tube that is welded into the larger tube. The assembly may also be heat treated after the inner tube is installed. The threads are preferably formed on the outer tube after the inner tube is installed and the assembly is heat treated.
Conventional threading operations require that the pipe be rotated as threads are being cut into the end of the pipe. Threading speeds of 150 rpm or more are common. Relative movement between the inner and outer pipe bodies as the assembly is rotated can cause harsh vibration and other undesired mechanical movement that interferes with the threading process.
The relative radial position of the inner and outer tubes can be fixed by employing multiple rigid spacers in the annulus between the two tubes. One commonly employed technique is to provide a hinged metal stamping with raised ears to close the gap between the inner and outer tubes. Because of the use of sheet stock as the spacer material, the contacting ears form a flat surface engaging the inside surface of the surrounding tube.
Another technique employed to fix the inner and outer tubes is to employ a square stock that has been cut to short lengths and welded onto the outside surface of the inner pipe. Because the spacer is cut from square stock, the contact ends form a flat surface contacting the inside surface of the inner pipe.
In both of the described prior art spacing techniques, the design and volume of these spacers can significantly increase the heat transfer between the inner and outer conductors. Benefits of a more stable assembly for threading purposes are offset by the increases in heat transfer between the tubes.
The shape, size, and distribution of the centralizers also affect the assembly process of inserting the inner tube into the outer tube. Multilayer wrappings of insulating material are often disposed over the smaller diameter tube between the axially spaced centralizers. It is typical to insert the inner tube, equipped with centralizers and insulation material, into the larger tube while the inner and outer tubes are horizontally oriented. As this is done, the inner tube slides along the internal bottom wall of the outer tube. This sliding movement can damage the centralizers and the insulation material. Horizontal insertion may also be made difficult when the internal surface of the outer tubular body is coarse or irregular such that the centralizers and insulation are abraded or hang up on the surface irregularities.
Many of the problems associated with horizontal assembly of the inner and outer tubes may be avoided by orienting the two tubes vertically while the inner tube is inserted. While the vertical orientation technique can eliminate the described problems associated with dragging the inner tube along the internal bottom wall of the outer tube, vertical orientation introduces other assembly problems. In this regard, the relative axial position of the inner and outer tubular bodies must be closely controlled such that the mid-length of each conduit is coincident. Additionally, the inner tubular body is usually shorter than the surrounding outer body so that the lengthwise centering must be accomplished within the surrounding pipe.
The desired spacing could be achieved by welding temporary lugs or retaining members to the inner or outer pipe at appropriate locations to form a stop that would fix the inner pipe lengthwise within the outer pipe. Such a procedure would require an additional welding step and also introduces an additional point for the introduction of measurement errors.